Inkjet printers have become increasingly popular because of such advantages as low-noise pollution and low-running cost, and inkjet color printers capable of printing on plain paper have also been increasingly put on the market. However, it is significantly difficult for inkjet printers to satisfy all the requirements such as color reproducibility of images, abrasion resistance, light resistance, gas resistance, water resistance and drying property of recorded images, resistance to character feathering, resistance to bleed at boundaries between different colors (resistance to color bleed), image density uniformity at solid image parts (resistance to beading), double-sided printing property and ejection stability. Thus, in reality, an ink and a recording medium are selected in view of preferential properties in accordance with the intended use.
In general, inks used in inkjet recording contain water as a main component and additionally contain a colorant, and a wetting agent such as glycerin with a view to preventing clogging. As for colorants, water-soluble dyes which enable obtaining vivid colors are commonly used. Dyes are, however, inferior in resistance to light, gases and water, and images recorded with dyes on plain paper are far from providing satisfactory image quality. To solve the problem, attempts to improve these resistances, i.e., resistance to light, gases and water, are made by improving properties of inkjet recording paper having an ink absorption layer. But, the improvements have not yet reached sufficient levels.
Recently, pigments have become widely used as colorants because of their superior light resistance, gas resistance and water resistance. However, it is true that pigments are inferior in color developing property to dyes. This can be primarily explained by the fact that lights having different wavelengths and/or phases caused by multiple reflection inside pigment particles interfere with each other. That is, when pigment(s) are used as inkjet recording ink(s), a reduction in color saturation occurs on plain paper, and a reduction in glossiness occurs on inkjet recording paper. To solve the problems, attempts are made to improve color saturation by making finer pigment microparticles and to improve glossiness of pigment particles by covering the particles with polymer. Despite these improvements, there still exist a difference in color developing property between pigments and dyes.
Particularly in magenta inks, quinacridone pigments are widely used for their superior color developing properties and light resistance. Quinacridone pigments, however, are somewhat more reddish in color than the standard magenta color (Japan color Ver. 2) used in printing. This tendency has become increasingly marked as an image is printed with higher image density. When an attempt is made to yield a vivid color with high image density, a resultant color inconveniently becomes more reddish. However, at the present time, there are no pigments that can achieve a color comparable to the standard magenta color with high color saturation through the use of one pigment alone. Recently, attempts to obtain high magenta color saturation are made using a to solid solution of quinacridone pigment, but the desired magenta color saturation has not yet been achieved.
Patent Literature 1 and other documents propose an ink using a solid solution pigment. In most cases, use of solid solution pigments makes it possible to obtain vivid is colors as compared with inks which contain pigments composed of monocrystal, however, solid solution pigments are still inferior in color developing property to dyes.
Meanwhile, attempts are made in Patent Literature 2 through Patent Literature 6 to increase color saturation, which has been insufficient with use of one type of dye, by mixing two or more types of water-soluble dye including xanthene dyes, and to improve light resistance, which has been insufficient with use of one type of dye, by mixing dyes having higher light resistance. However, the light resistance of dyes is still insufficient, even if a plurality of dyes are mixed, it is incomparable with the light resistance of pigment inks at the present time.
As an ink that satisfies both excellent light resistance obtained from pigments and excellent brilliant coloring effect obtained from dyes, Patent Literature 7 proposes an ink produced by mixing Rhodamine B-based water-soluble dye and a pigment. With this method, the light resistance can be improved as close as that provided by using a pigment, however, when a penetration-type ink having low-surface tension is printed on plain paper, feathering and color bleed occur as in the case where a dye-based ink is used, resulting in incapability of obtaining an image with high-sharpness.
Also, an ink produced by mixing two or more types of different organic pigments is proposed. Patent Literature 8 proposes to adjust color tone and improve color saturation by adding from 0.01% to 1% of an organic pigment having a hue angle of 270 degrees to 340 degrees to a quinacridone pigment. However, even if such a small amount as 0.01% to 1% of the pigment is added, a change in color tone and a further improvement in color saturation can be little expected. In contrast, when a lake pigment (B) is added to a solid solution pigment (A) at a mass ratio (B/A) within the range of 0.1 to 5, as in the present invention, it is possible to adjust color tone and improve color saturation. Patent Literature 9 proposes an inkjet recording ink produced by mixing a quinacridone pigment and a pigment(s) of red, violet, orange, etc. in color, but the proposal is intended to achieve inks for use in color filters with more transparent and more vivid color. Thus, the object of the proposed ink differs from that of the magenta ink of the present invention.
The present inventors proposed before in Patent Literature 10 a technique relevant to an ink composition containing colored polymer fine particles in which an organic pigment (solid solution pigment), an oil-soluble dye and an inorganic pigment are covered with a polymer, but the ink proposed by the technique differs from an ink according to the present invention in which an organic pigment (specific solid solution pigment) and a lake pigment are mixed.    Patent Literature 1 Japanese Patent (JP-B) No. 3556835    Patent Literature 2 Japanese Patent (JP-B) No. 3829419    Patent Literature 3 Japanese Patent Application Laid-Open (JP-A) No. 9-286167    Patent Literature 4 Japanese Patent Application Laid-Open (JP-A) No. 2001-192591    Patent Literature 5 Japanese Patent Application Laid-Open (JP-A) No. 2002-235020    Patent Literature 6 Japanese Patent Application Laid-Open (JP-A) No. 2003-238875    Patent Literature 7 Japanese Patent Application Publication (JP-B) No. 60-045669    Patent Literature 8 Japanese Patent (JP-B) No. 3670148    Patent Literature 9 Japanese Patent Application Laid-Open (JP-A) No. 2003-292812    Patent Literature 10 Japanese Patent Application Laid-Open (JP-A) No. 2008-255241.